Protein dosage of the lldPRD operon is correlated with RNase E-dependent mRNA processing.

ABSTRACT The ability of Escherichia coli to grow on l-lactate as a sole carbon source depends on the expression of the lldPRD operon. A striking feature of this operon is that the gene encoding the transcriptional regulator (LldR) is located between the genes encoding the permease (LldP) and the dehydrogenase (LldD). In this study, we report that the dosages of the LldP, LldR, and LldD proteins are not modulated on the transcriptional level. Instead, modulation of the protein dosage is correlated primarily with RNase E-dependent mRNA-processing events that take place within the lldR mRNA, leading to the immediate inactivation of lldR, to differential segmental stabilities of the resulting cleavage products, and to differences in the translation efficiencies of the three cistrons. A model for the processing events controlling the molar quantities of the proteins in the lldPRD operon is presented and discussed. IMPORTANCE Adjustment of gene expression is critical for proper cell function. In the case of polycistronic transcripts, posttranscriptional regulatory mechanisms can be used to fine-tune the expression of individual cistrons. Here, we elucidate how the protein dosage of the Escherichia colilldPRD operon, which presents the paradox of having the gene encoding a regulator protein located between genes that code for a permease and an enzyme, is regulated. Our results demonstrate that the key event in this regulatory mechanism involves the RNase E-dependent cleavage of the primary lldPRD transcript at an internal site(s) located within the lldR cistron, resulting in a drastic decrease in the amount of intact lldR mRNA, in differential segmental stabilities of the resulting cleavage products, and in differences in the translation efficiencies of the three cistrons.